INTERNAL FRICTION IN ROCKS 599 



White dolomite, Cockeysville, Maryland, U.S.A. 



Steatite (" Albarine ") , Virginia, U.S.A. 



Slate, New Rockland, Province of Quebec, Canada. 



Sandstone, Cleveland, Ohio, U.S.A. 



Granite, Baveno, Italy. 



Olivine diabase, Sudbury, Province of Ontario, Canada. 



For the purposes of comparison experiments were also con- 

 ducted with metallic copper and metallic lead. 



Detailed petrographical descriptions of these rocks, with the 

 exception of the alabaster, dolomite, steatite, and slate, have been 

 given in a former paper. 1 It is necessary here, therefore, to refer 

 briefly to the character of these four rocks only. 



Alabaster, Castelino, Italy. — Under the microscope the rock is 

 seen to be composed of an aggregate of small grains of gypsum 

 which are clear, colorless, and approximately equal in size. The 

 individual grains display a tendency to elongation in one direction, 

 thus giving the rock a very faint foliation. The columns of ala- 

 baster used in the experiments were cut from a single uniform block 

 of this rock in such a manner that their longer axes were parallel 

 to this indistinct foliation. 



Dolomite, Cockeysville, Maryland, U.S. A .• — This is a rather fine- 

 grained, white, granular dolomite, very pure in character and uni- 

 form in composition, containing CaC0 3 and MgC0 3 in almost 

 exactly their molecular proportions. It presents the appearance 

 of a white marble and is extensively quarried as such. Thin sec- 

 tions of the rock, when examined under the microscope, show that 

 it is composed of a mosaic of grains of the mineral dolomite, more 

 or less irregular in shape and varying somewhat in size. Between 

 crossed nicols, they present a uniform extinction or show only the 

 faintest strain shadows. They are very seldom twinned. 



Steatite, Virginia, U.S. A . — This steatite is placed on the market 

 under the name of "albarine." The columns employed in the 

 experiments were cut from a perfectly uniform slab of this rock 



1 "An Investigation into the Elastic Constants of Rocks More Especially with 

 Reference to Their Cubic Compressibility," by F. D. Adams and E. G. Coker, The 

 Carnegie Institute of Washington, 1906; see also American Journal of Science, XXII 

 (August, 1906). 



